Advances in technology in the electronics industry continually drive manufacturers to reduce the size of finished products while, at the same time, increasing their power. This is certainly true with respect to notebook and palmtop computers as well as with cellular telephones and camcorders. Desk top computers today are as powerful as those which occupied an entire room years ago. Telephones now fit in a shirt pocket.
Nevertheless, there are sectors of the industry, for example, in the printed circuit board fabrication business and the assembly shops that attach components, such as packaged integrated circuit devices to the board, which have reached their limits using conventional approaches. The two major obstacles being the application of solder mask between surface mount pads and the printing of solder paste on those pads.
Typically, a printed circuit board having thereon both fine and coarse pitch copper contact patterns is coated with a permanent solder mask whose purpose is to allow soldering only to the exposed copper patterns. In many cases, it is desirable to image a solder mask dam or web at sites between surface mount lands to act as an isolating barrier during subsequent solder paste printing and reflow. This dam or web prevents shorts from forming. The solder mask must be thick enough to provide a dam into which an adequate volume of solder paste can be deposited by screen printing for subsequent reflow.
On coarse pitch products, about 0.016" (16 mil) or higher, this does not present a problem as the application and imaging of the solder mask is relatively easy. There is a sufficient gap between lands on the board to enable resolution of this web. When the pitch becomes finer, it becomes increasingly difficult to align (register) and develop these features. In many cases this obstacle is insurmountable using conventional technology and different approaches need to be used.
The basic processes involved in attaching surface mount components to a printed circuit board consists of the following: 1. Paste Printing; 2. Component Placement; and 3. Solder Reflow. The largest number of defects associated with this process, approximately 70%, are directy attributable to the printing of solder paste. For this as well as other reasons, several processes have recently been developed which apply a sufficient amount of solder to the board prior to the assembly stage which significantly simplifies the process and reduces the associated defects. However, processes such as Precision Pad Technology (PPT.RTM.), Optipad.RTM., and Sipad.RTM. all require a solder mask dam or web between the lands for fine pitch applications in order to isolate the features and contain the solder.
Surface mount technology (SMT) is now routinely used to attach packaged integrated circuit devices to printed circuit boards. In one form, the practice involves the use of solder paste selectively deposited onto copper contacts of the printed circuit board through a stencil patterned with openings corresponding to the board contact locations. The solder paste is screen deposited in patterns on the printed circuit boards using the stencil as a mask and a doctor blade to squeegee the solder paste through the holes in the stencil. When the stencil is removed, the solder paste remains on the printed circuit board contacts.
Since the solder paste is typically 50% flux by volume, with the other 50% being particles of solder, the paste is also used to hold the component terminals in place during the solder reflow step which follows. The eutectic low melting point solder (63% tin, 37% lead-63/37) normally used permits reflow and concurrent bonding of the component terminals to the printed circuit board contacts at a temperature below 250 deg. C. compatible with the glass transition capabilities of flame retardent level 4 (FR4) printed circuit board materials.
The spacing of the leads for packaged integrated circuit components typically exhibit a pitch no finer than 16 mils. This capability is consistent with conventional SMT processes using screen deposited solder paste to hold and connect packaged integrated circuit devices to printed circuit boards.
Photolithographic processes used to form conductive contact patterns on modern printed circuit boards have the capability to create correspondingly fine pitch patterns. Unfortunately, attempts to screen solder paste in the fine pitch patterns characterizing the more advanced packaged integrated circuit devices in the neighborhood of 10 mil pitch have been frustrated by solder mask/solder paste problems.
In the absence of an effective solder paste screening process for depositing solder onto fine pitch printed circuit board contacts, other techniques have been employed. One approach uses masking and electroplating to deposit the solder. This process includes the formation of a photolithographically defined mask, an electroplate bath deposition of low melting solder on printed circuit board contacts not covered by the mask, a removal of the mask, and a reflow of the electroplated solder. The process involves numerous steps and has associated high cost.
Another approach involves the injection of molten solder through a dispensing head with a mask corresponding to the copper contact pattern of the printed circuit board. Unfortunately, the molten solder dispensing head is very expensive, requires a distinct mask for each different device footprint, and dispenses the solder to the contacts of only one die location at a time. An additional approach which reportedly has been used involves coating the entire board with molten solder thereby filling the wells and then blowing off the solder remaining on the solder mask. Of course, subjecting an entire board to the temperature and aggressiveness of molten solder is not an optimum solution to these problems.
The PPT.RTM. process has resulted in several U.S. patents being granted to Holzmann with the following U.S. Pat. Nos. 5,310,574; 5,395,040; and 5,403,671; as well as European Patent No. EP 0 640 271 B1 which contains essentially all significant features of the three prior U.S. patents. These patents teach the use of a mesh pressed against the circuit board with subsequent reflow whereby the mesh insures filling the well and shaping the formed solder with an imprint of the mesh on its top. Furthermore, the mesh forces excess solder, including solder balls, onto the upper surface thereof, thereby eliminating the possibility of short circuits caused by the excess solder . . . all the wells are filled. This excess is easily removed from the mesh. The other two processes, i. e., Optipad.RTM. and Sipad.RTM., flatten the top surface of the solder deposit. All three of these processes suffer from the need for a solder mask for fine pitch applications, which mask has severe resolution problems when defining a fine pitch well for deposition of solder.
Other relevant prior art can be found in U.S. Pat. No. 5,492,266 granted to Hoebener wherein is disclosed a method and product for fine pitch solder deposits using a stencil to screen the solder paste leaving the stencil on the board on top of the solder mask during a reflow step. This method is suited to the fabrication of populated printed circuit boards having fine pitch devices including flip-chip devices, connected on a board including conventional coarse pitch surface mount components.
As Hoebener points out in his patent, however, there are several weaknesses in this method. For example, it is stated that, "Tests to date have confirmed that the shape of the hole has an effect on the volume and formation consistency of the low melting point solder deposited onto contact 2. However, the optimum shape has yet to be defined, and is likely to be related to the material used for the stencil, the thickness of the stencil, the size of the contact, and the composition and rheology of the solder paste." (emphasis added)
Furthermore, "The use of a common stencil for both the fine and coarse pitch contacts may result in less than normal solder volume for the coarse pitch contacts. However, it is anticipated that any such variations or problems can be overcome through process optimization."
In addition, "The solder formed on the coarse pitch contacts may not have to be flattened in preparation for component placement." This flattening requirement itself can be a substantial problem since it involves additional equipment and time, and runs the risk of producing a squeeze-out of excess solder, which solder can produce short circuits, and the need to clean this excess off.
Another alternative, " . . . involves the practice of the invention in which the stencil is not removed following reflow of the solder paste. This practice will likely involve the screening of paste for both fine and coarse pitch contacts using the common stencil, and the use of the flux to hold the components during final reflow step 36 . . . " One final point is that in the Hoebener method, solder mask is a requirement.
One recent relevant disclosure appeared in Electronic Packaging & Production, May '97, in an article entitled, "Forming BGAs with Solder Paste", Brutovsky, et al, of IBM, pp.57-61, wherein a top and bottom stencil are employed. The authors point out that existing paste stenciling operations produce highly repeatable solder volumes at high throughput rates. Also, solder purchased in paste form is much less expensive than that purchased as discrete balls of tightly controlled size conventionally used in BGA (ball grid assembly). Paste volume is a function of stencil thickness and aperture size. Stencil thickness is the key variable for controlling paste volume and resulting BGA ball size.
To avoid the problem of solder paste sticking to the aperture walls and pulling away from the laminate, these inventors use two different stencils. Their top stencil being fixed in an SMT screen printer is removed after screening, whereas their bottom stencil, made of magnetic steel with chrome plating, stays with the laminate through the solder paste reflow operation and is only removed thereafter. A special workholding fixture having magnets embedded therein holds the bottom stencil firmly in place against the PCB laminate.
Due to the size of this "sandwich", early designs were too massive to be heated adequately in existing reflow ovens; therefore, to reduce thermal mass, a window-frame-like design with special cutouts was used. The stenciling and reflow operation provide a consistent volume of solder to each BGA pad.
Since this method of depositing solder results in consistent solder volumes, it is less desirable when shaping solder deposits simultaneously on fine and coarse pitch contacts which require the deposit of variable solder volumes. Where varying volumes are necessary, the instant invention solves this problem as well, without the use of reflow ovens, and without the need for complex magnetic fixtures and stencils which appear to be suitable only for very high volume production.
In the context of this known technology, there remains the need for a process, and product formed thereby, which will deposit and shape solder on a fine pitch printed circuit board contact pattern within the framework of conventional screened solder paste deposition processes while providing solder volumes on both coarse and fine pitch contacts simultaneously adequate to connect conventional surface mount devices and other packaged integrated circuit device components in the presence or absence of a solder mask.